This invention relates to a process for obtaining broad bimodal or multimodal molecular weights.
It is known that some metallocenes such as bis(cyclopentadienyl) titanium or dialkyl zirconium in combination with aluminium alkyls as co-catalyst form homogeneous systems which are useful for the polymerisation of ethylene. German patent 2,608,863 describes the system of dialkyl bis(cyclopentadienyl) titanium with trialkyl aluminium.
The controlled hydrolysis of aluminium alkyls leads to the formation of species which include Alxe2x80x94O bonds called aluminoxanes. Kaminsky et al. (Adv. Organomet. Chem. 18, 99, 1980) have demonstrated that aluminoxanes in combination with chlorinated metallocenes produce very active catalytic systems for the polymerization of ethylene. The need to reduce or eliminate aluminoxanes, as these are highly flammable, and are used in large proportions in relation to the metal in polymerization reactions, led to the use of bulky boron compounds which replaced the aluminoxane in stoichiometric molar relationships with respect to the metal of the metallocene, as described in the patents by Turner, EP 277004 and Ewen et al., EP 426637. The metallocenes used in these patents should be dialkyl derivatives, as boron compounds do not have any alkylating capacity. Alkylated metallocenes are highly unstable with water, oxygen and air, with the result that a large proportion of the metallocene present becomes deactivated before polymerization starts, as it is used up in reacting with the impurities which are present in the medium which have not been eliminated previously. This does not happen when aluminium compounds such as aluminoxane are used, as these act to eliminate impurities from the reaction medium avoiding deactivation of the metallocene.
The use of metallocenes in combination with aluminium alkyls and aluminoxanes and boron compounds has been described by Razavi et al. in CA 2,027,144 for the polymerization of syndiotactic polyolefins, with the objective of using small quantities of aluminium compound to remove the impurities and alkylate the metallocene. More recently, Chien et al. (Macromol. Chem. Macromol. Symp., 66, 141-156, 1993) have also used small quantities of aluminium alkyls, generally triethyl aluminium or trisobutyl aluminium, with the same object of removing impurities and alkylating the chlorinated metallocene, finally adding the boron compound as an activator for the reaction. This system makes it possible to use chlorinated metallocenes, which are stable in air, reducing the amount of aluminium compounds in the catalyst system.
On the other hand these authors comment that the use of these alkyl aluminiums together with these boron compounds is not obvious, because secondary xcex2-H elimination reactions, or reactions wherein the boron compound may interfere with the catalytic system, can occur. Many patents have appeared in the last year claiming the use of these mixed co-catalyst systems, e.g. Mitsubishi Petrochemical Co. Ltd. (EP 574258, JP 5295021); Tosoh Corp. (JP 5339316, JP 5310829, JP 5301919, EP 570982, JP 5255423, JP 5239140); Idemitsu Kosan Co. Ltd. (JP 5331219, WO 9324541, JP 5320258-60, JP 5320245-48, JP 5271339, JP 5262823, JP 5262827, JP 5043618); Mitsui Toatsu (JP 5155927, JP 5140221). All these patents claim the use of a smaller quantity of aluminoxane as advantages in the use of mixed co-catalysts, and greater activity and better properties for the polymer, such as e.g. improved processability, high heat resistance, good transparency, etc.
The term xe2x80x9cbimodal or multimodal molecular weight distributionxe2x80x9dmeans that two or more peaks of different molecular weight can be seen in chromatograms obtained by gel chromatography (GPC), which represent the molecular weight as a function of the relative proportion of polymer having a specific molecular weight.
Polyolefins which have bimodal or multimodal molecular weight distributions, like polyethylene, can be transformed into articles by extrusion moulding, hot shaping, spin moulding, etc., and in general their most common applications are for tubing, films and articles made by the blowing technique. The advantages over other polyolefins which do not have multimodal molecular weight distributions are easier and faster processing with a reduced energy requirement. In addition to this, bimodal polyolefins show less flow disturbances when molten and are preferred on account of their improved properties such as a better balance between rigidity and resistance to cracking through environmental agents.
There are in the literature different ways to obtain polymers with broad bimodal or multimodal molecular weight distributions, such as the use of reactors in series, performing polymerization with a catalyst in the absence of hydrogen (which acts as a chain transfer agent reducing the molecular weight) in the first of these reactors, yielding a high molecular weight fraction, and then passing the polymer through a second reactor in which the polymerization is continued in the presence of hydrogen to produce another fraction of lower molecular weight. Examples of this process are the patents by Morita et al., U.S. Pat. No. 4,338,424, Kuroda et al. U.S. Pat. No. 4,414,369, Rafaut U.S. Pat. No. 4,703,094, Kato et al., U.S. Pat. No. 4,420,592. These methods to produce polymers with a multimodal molecular weight distribution are consequently expensive, complicated and require a longer time, and require more complex plant.
Another way of obtaining these multimodal polymers is through the use of a single reactor, but using mixtures of two or more different catalysts. Each catalyst produces a polymer having a different molecular weight and molecular weight distribution according to the polymerization reaction rate constants for each catalyst, or as a result of differences in reactive behaviour to different monomers and/or the presence of H2, e.g. Ewen et al., EP 128045, use mixtures of different metallocenes, with different rates of propagation and reaction with respect to the same monomer. In another patent, EP 128046, Ewen et al. use mixtures of metallocenes with different reactivities to different monomers. With the same object Stricklen et al., U.S. Pat. No. 4,939,217 and U.S. Pat. No. 5,064,797, on the other hand use two or more metallocenes alkylated with MAO, with a different behaviour to H2, obtaining polymers with different molecular weights. In another patent, WO 9215619, Stricklen et al. effect polymerization in two stages, the first being a homopolymerization in the presence of H2 and the second a copolymerization in the presence of H2. Ewen et al., EP 310734, use mixtures of two chiral stereorigid metallocenes for the same purpose. However, it is difficult to control the feed of the two catalysts, and, as the particles are not of uniform size, segregation processes occur during storage and transfer of the polymer resulting in non-homogeneous polymer mixtures.